# Chemical Kinetics

#### Law of mass action:

The theoretical dependence of rate of reaction is explained by the law of mass action.

The law states that the rate of a reaction is proportional to the product of the concentration of the reactants raised to their appropriate concentrations terms.

#### The rate law:

The actual dependence of rate of reaction is determined experimentally and is called rate law.

#### Collision Theory:

The rate of reactions depend on two factors

The number of collisions per unit time between reacting species

The fraction of these collisions that are successful in producing a new molecule.

#### Rate:

The rate of a reaction may be defined as the change in the concentration of any one of the reactants or products per unit time.

Unit of rate mol/L/Time

#### Average rate:

Total change in concentration by total time taken is known as average rate.

#### Instantaneous rate:

The rate of change of concentration of any one of the reactants or products at that particular instant of time can be expressed as,

#### Graphical method for determination of rate:

The rate can be determined by finding the slope of the tangent to the curve at the point corresponding to that instant of time.

#### Examples of differential rates:

Write differential rates for the reaction:

Dependence of rate of reaction on the concentration of the reactants:

#### Rate Law and Order of reaction

The dependence of the rate of a reaction on concentration is seen form the law of mass action. It gives a theoretical dependence of the rate of the reaction on concentration of reactants.

[Refer Chemical equilibrium for Law of mass action

aA + bB $\rightleftharpoons$ cC + dD

$Rate_f = k^1[A]^a[B]^b and Rate_b = k^{11}[C]^c[D]^d$

At equilibrium Rf = Rb

$Rate=k\frac {[C]^c[D]^d}{[A]^a[B]^b}$

The above is the sum of law of mass action]

#### The rate law is determined experimentally:

In a multi step reaction the rate depends on the slowest step that is the rate-determining step.

Here ‘k’ is the rate constant or the specific reaction rate or the velocity constant.

‘k’ can be defined as the rate of the reaction when the concentration is unity.

#### Order

It can be defined as the sum of the exponents of the concentration terms of the rate law

Or

It may be defined as the sum of the powers to which the concentration terms are raised in a rate law expression. This is an experimentally determined factor.

For the reaction

Rate = k [A]x [B]y

The order ‘n’ of the reaction = ‘x’ + ‘y’ where ‘a’ and ‘b’ may or may not be equal to ‘x’ and ‘y’

Page 2 Chemical Kinetics Continued

$X^2$
$\frac{p}{q}$
$K_{eq} = \frac{[C]^c[D]^d}{[A]^a[B]^b}$
$Rate_f = k^1[A]^a[B]^b$
$ax^2 + bx + c = 0$